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<script>
  /* 
    ++n 与 n++ 不参与表达式时结果都是一样的
   */
  let n = 1;
  n = n + 1; // 等于++n

  let m = 2;
  let i = m + ++n; //相当于 m + (n=n+1)
  let j = m + n++; //相当于 (m + n)++
</script>

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